The present invention relates to an apparatus for removing soot or particulates from the exhaust gases of an internal combustion engine, especially a Diesel engine.
Applicant's copending U.S. patent application, Ser. No. 069,609, filed July 2, 1987, discloses an apparatus of this general type, which includes a metallic housing with inlet and outlet connectors for the exhaust gas stream, and a filter insert that is disposed in the housing and contains a nonmetallic, high-temperature-resistant filter means in the form of a granulate fill that is disposed in a fixing element, wherein the filter insert in the filter housing separates a crude or untreated-gas chamber from a purified-gas chamber, and an electrical current source is used for burning off, in a controlled manner, the soot deposited on the filter means. This copending application Ser. No. 069,609 is incorporated into this disclosure by this reference thereto.
According to the aforementioned application Ser. No. 069,609, the soot itself acts as an electrical conductor of heat. Surface filters (for example yarn or fabric filters) or deep bed filters (for example loose material or granulate filters) can be used.
Surface filters run the risk of receiving combustion residues, such as ash. On the other hand, such ash particles merely increase the volume of granulate filters, provided that they have not previously been discharged by the exhaust gas that flows through the looser granulate structure. However, granulate filters produced with the normal loose fill generally necessitate a large volume of loose material and therefore a considerable weight, which (often) makes them unsuitable for use in vehicle engines.
The reason for the large volume required by loose material filters is the relatively long average path which the respective soot-laden exhaust gas has to cover in the granulate to achieve a certain desired probability of separation of the soot contained.
The abovementioned "separation path length" is directly dependent on the average granule size if the effect of the exhaust gas speed in the fill is disregarded. This is related to the fact that an increase in the diameter of the granule particles also means an increase in the cross section of the average free microscopic passage for the exhaust gas; in the long run, this causes a drop in the separating capacity, which can then be improved only with a greater fill height (corresponding to a greater separating path length).
The conclusion reached, involving selecting granule particle sizes having smaller diameters when dimensioning the filter, has limits in that the accompanying requirement for a reduction in the gas throughout (to avoid discharge losses) leads to an enlargement of the perforated exhaust gas intake base.
A smaller fill height and, at the same time, an enlarged inlet surface and smaller granule diameter, follow a law that does not lead to the expected reduction in volume and therefore in weight, which is a condition for use of the loose material filter in vehicles.
One way of enabling lower weight granulate structures to be used for producing a deep bed filter design involves confining the fill.
The basic idea is as follows: With (conventional) granulate filters, the permitted volumetric flow per unit of passage area is largely dependent on a specific value of the ratio between the two forces acting on the individual grain of loose material during operation. This includes the tractive force (acting against gravity) which acts on the grain surface, is dependent on the flow rate, and counteracts the weight of the same grain. If the tractive force exceeds the weight (force ratio exceeds 1), the grain is discharged in an undesirable manner. On the other hand, if a discharge barrier in the form of a screen is provided, this barrier allows considerably higher flow rates inside the filter up to a pressure drop value which can just be tolerated as a counterpressure component of the exhaust gas stream from the engine. It can easily be seen that such a filter is distinguished by a comparatively low loose material requirement, and is thus suitable for vehicles.
The object of the invention is to provide a relatively simple means for confining loose material which works particularly well, is reliable in operation, and produces a very high degree of soot separation in the filter using a soot filter of the aforementioned general type.